This invention relates to a method of clarifying liquids and, in particular, to a filtration system for unattended clarification of varying influent loads.
More specifically, this invention relates to a filtration system utilizing primary and secondary filters with storage and dilution reservoirs to provide a constant quality effluent from an influent which varies both in quantity of discharge and percentage of solids concentration.
Filtration systems are used in many applications wherein it is desired to clarify liquids by removing solids and chemical contaminants prior to discharging the liquid, for example, into sewers. Certain types of applications require that a liquid be clarified by the filtration system in order that it be continuously reused without discharge into sewage lines or subjected to a costly method of ultimate disposal. In these applications, referred to as closed loop systems, the liquid must be reclaimed either due to environmental pollution control or cost constraints. Such a closed loop filtration system is utilized in many applications wherein the cost of the liquid clarified in such that it would be uneconomical to dispose of the liquid, or the liquid is of such a nature that the final disposal costs require that the liquid be utilized for many cycles prior to being expended.
In certain other types of applications wherein the liquid is discharged for disposal such as into sewage lines, referred to as an open loop system, it is necessary for environmental pollution control that certain materials conveyed or suspended in the liquid be removed therefrom prior to discharge. The contaminants or suspended materials removed from the liquid must be collected for disposal in the most economical manner.
In either an open loop system or a closed loop system, the influent may vary both as to the quantity being discharged to the filtration system and/or the percentage of solids contained in the influent. If such a varying influent is coupled directly to the filtration system, the system must be continually monitored by an operator to handle the varying loads of influent and must be designed such that the maximum load can be passed through the filtration system. Since the quantity of influent is variable such a filtration system would be overdesigned and, therefore, would not be operating at its maximum efficiency except during peak loading periods.
In an attempt to eliminate this problem, storage reservoirs have been used in connection with such filtration systems so that the surges of high volume liquid can be stored within the reservoir or holding tanks to provide a more constant quantity of influent through the filtration system. However, such systems are only capable of passing the influent through the filtration system in a single cycle resulting in inconsistent uniformity of the discharged filtrate due to variable solids concentrate in the influent. The varying solids concentration frequently results in unclarified liquid bleeding through the filter elements.
Another problem associated with these prior art systems is that the heavy solids concentration in the influent to the filtration system causes the filter elements to blind or clog. The blinding or clogging of the filter element precoat results in a shortened filtration cycle and necessitates frequent replacement of the filter cake or precoat formed on the filter elements. The frequent replacement of the expended filter cake results in increased material costs, e.g., the filter cake forming medium, and causes increased unit cost due to the necessity for larger filter area and the spacing required thereby in order to handle the maximum concentration of solids in the influent.
Since none of these problems have heretofore been satisfactorily solved through the use of a pressure precoat type filter, the only somewhat satisfactory system for handling variable influent has been a rotary vacuum filter. Whenever the concentration of solids in a comtaminated liquid exceeds approximately three to five per cent, the rotary vacuum system has heretofore been the only system which has been somewhat commercially successful.
A rotary vacuum filter utilizes a rotating drum of filter cloth which serves as the filter septum for forming or supporting a filter cake thereupon. The filter cake bearing drum is submerged approximately one-third of its surface area in the contaminated liquid and continuously rotated. The precoat or filter cake formed on the filter septum is usually three to six inches in depth and as the unclarified liquid is being passed therethrough to be discharged from the drum interior, the precoat or filter cake traps the solids material clogging the filter and precluding further filtration. Therefore, these units utilize a knife or scraper positioned adjacent the filter drum such that the knife or scraper automatically advances into contact with the filter cake and removes a given cake thickness during each and every revolution of the drum. While such a system increases utilization of the filter drum resulting in a longer filtering cycle of operation before having to have the filter cake removed, the continuous scraping and removal of the outer surface of the filter cake increases the solids contents of the contaminated liquid progressively decreasing the time period in which the contaminants will again blind or clog the filter surface.
Another problem associated with the rotary vacuum filters is that as the clarified liquid is drawn through the filter cake into the interior of the drum through the submerged portion and air is drawn through the filter cake through the exposed portion, the air-exposed filter cake will crack or shrink due to the air passing therethrough resulting in poor filtration when the cracked filter cake is submerged in the contaminated liquid during rotation of the drum. The cracks in the filter cake will allow the unclarified liquid to pass through the filter contaminating the clarified filtrate.
Since the utilization of the prior art filtering device as described above involves either an incomplete and unsatisfactory filtering of the influent or requires costly expenditures both as to the equipment and materials utilized, none of these devices are completely satisfactory for general use in handling variable influents.